Placement Usage

The placement service enables other projects to track their own resources.
Those projects can register/delete their own resources to/from placement
via the placement HTTP API.

The placement service originated in the Nova project. As a
result much of the functionality in placement was driven by nova’s
requirements. However, that functionality was designed to be sufficiently
generic to be used by any service that needs to manage the selection and
consumption of resources.

Two processes, nova-compute and nova-scheduler, host most of nova’s
interaction with placement.

The nova resource tracker in nova-compute is responsible for creating the
resource provider record corresponding to the compute host on which the
resource tracker runs, setting the inventory that describes the quantitative
resources that are available for workloads to consume (e.g., VCPU), and
setting the traits that describe qualitative aspects of the resources (e.g.,
STORAGE_DISK_SSD).

If other projects – for example, Neutron or Cyborg – wish to manage resources
on a compute host, they should create resource providers as children of the
compute host provider and register their own managed resources as inventory on
those child providers. For more information, see the
Modeling with Provider Trees.

The nova-scheduler is responsible for selecting a set of suitable
destination hosts for a workload. It begins by formulating a request to
placement for a list of allocation candidates. That request expresses
quantitative and qualitative requirements, membership in aggregates, and in
more complex cases, the topology of related resources. That list is reduced and
ordered by filters and weighers within the scheduler process. An allocation
is made against a resource provider representing a destination, consuming a
portion of the inventory set by the resource tracker.